The adhesive strength of an adhesive has dependency upon an applying amount, namely, a thickness of an adhesive layer to be formed, and in pressure sensitive adhesive sheets which are used for labels, tapes, and the like, the thickness of the adhesive layer is set up in conformity with a variety of purposes. However, there are not many examples of making the thickness thinner than 10 μm for the purpose of producing stable adhesive strength.
In recent years, in electronic appliances, optical appliances, and the like, reduction of thickness in products is desired, and pressure sensitive adhesive sheets which are used for joining or temporary bonding at the time of processing or the like of members constituting an electronic appliance or an optical appliance are also similarly desired to be thinned.
As for reduction of thickness in the adhesive layer in a pressure sensitive adhesive sheet which is used for joining or temporary bonding at the time of processing of members of an electronic appliance or an optical appliance, there have been made a variety of proposals. Though there may be the case where reduction of thickness is conducted as a measure for lowering the adhesive strength, there may also be the case where even when the adhesive layer is reduced in thickness, a lowering of the adhesive strength is not occasionally desirable.
For example, in Patent Document 1, removable properties of a protective film for a liquid crystal screen or the like are enhanced by controlling the thickness of an adhesive layer composed of a two-pack crosslinking type acrylic pressure sensitive adhesive to from 2 to 10 μm (from 2.5 to 3 μm in the Examples), namely, the adhesive strength is lowered by means of reduction of thickness.
In Patent Document 2, peeling-off from a laminated circuit board is made easy by controlling the thickness of an adhesive layer composed of a silicone based pressure sensitive adhesive from 0.5 to 20 μm (1.0 μm in Examples), namely, the adhesive strength is lowered by means of reduction in thickness.
In Patent Document 3, an object thereof resides in the matter that even when the thickness of an adhesive layer is reduced to a level of from 2 to 10 μm (3 μm in Examples), the bonding reliance on a liquid crystal display or the like is not lowered by using a rubber-based pressure sensitive adhesive, namely, even when the adhesive layer is reduced in thickness, the adhesive strength is not lowered.
In Patent Document 4, an object thereof resides in the matter that even when the thickness of an adhesive layer is reduced to a degree of from 2 to 10 μm (from 2 to 7 μm in the working examples), the bonding reliance on an optical member is not lowered by using a rubber-based pressure sensitive adhesive, namely, even when the adhesive layer is reduced in thickness, the adhesive strength is not lowered.
In Patent Document 5, removable properties in a wafer processing step are enhanced by controlling a total thickness of two adhesive layers composed of a crosslinking type acrylic pressure sensitive adhesive or the like to less than 10 μm (from 3 to 7 μm per one layer in Examples), namely, the adhesive strength is lowered by means of reduction in thickness.
Furthermore, in Patent Document 6, a pressure sensitive adhesive type optical film using a pressure sensitive adhesive containing a (meth) acrylic polymer having a large weight average molecular weight and composed of a heterocyclic ring-containing acrylic monomer and (meth)acrylic acid, a hydroxyalkyl (meth)acrylate, and an alkyl (meth)acrylate, and a crosslinking agent, and the like, are disclosed; it is described that the thickness of an adhesive layer is from 1 to 15 μm; and an object thereof resides in the matter that even when the adhesive layer is reduced in thickness, the adhesive strength is not lowered. However, in Examples of Patent Document 6, though it is described that the adhesive strength of the adhesive layer in a thickness of 5 μm is about 6N/25 mm, the adhesive strength in the case that the adhesive layer is more reduced in thickness is not described.
On the other hand, also in general-purpose acrylic pressure sensitive adhesives such as those for skin-sticking use, label use, etc., and the like, when reduction in thickness is conducted, a lowering of the adhesive strength is remarkably observed, too especially in a region of less than 5 μm.
In consequence, in general-purpose pressure sensitive adhesive sheets, any improvement for aiming to especially make the adhesive layer thin to a degree useful for the foregoing electronic appliances and the like has not been proposed yet.
For example, in Patent Document 7, the thickness of an adhesive layer formed of an acrylic pressure sensitive adhesive or a rubber based pressure sensitive adhesive in a transdermal preparation sheet is controlled to from 10 to 80 μm; however, it is not aimed that even when the adhesive layer is reduced in thickness, the adhesive strength is not lowered.
In addition, in Patent Document 8, there is proposed an easily applicable pressure sensitive adhesive sheet which is suitable as a large-sized pressure sensitive adhesive sheet for decoration, such as those for a signboard, a wall, a vehicle, etc. However, the thickness of an adhesive layer is from 7 to 100 μm, and it is not aimed that even when the adhesive layer is reduced in thickness, the adhesive strength is not lowered (estimated about 30 μm in the Examples).
Furthermore, Patent Document 9 discloses a pressure sensitive adhesive composition for general-purpose pressure sensitive adhesive sheet containing a urethane resin prepared by using two kinds of chain extenders having three or more functional groups and an acrylic polymer. However, in the Examples, an initial adhesive strength and removable properties are merely measured in a thickness of the adhesive layer of 25 μm, and it is not aimed that even when the adhesive layer is reduced in thickness, the adhesive strength is not lowered.